There seems to be a lot of questions about cavitations and what causes it? What can I do about it? Why does my boat loose power when I turn real sharp? Then, the most asked question, why does my engine rev up without the boat moving?

I’ve explained this to a few people but because it seems that most members have noticed it, I’m going to shed a little light on if in a thread.

A boat with an outboard motor uses a propeller to create forward thrust. What ever the speed of the propeller and what the degree is, determines the speed of the boat. A propeller “chops” at the water, digging its way into the water to push itself along its forward path.

A jet boats propulsion system has no comparison with an outboard motors principle of thrust, except that they both are sources of power. From there, it ends.

I’ve done my best to make a little drawing with paint (I’m not an artist, so bare with the drawing) to try and help explain the theory and principles of jet boat thrust.

An axial flow jet pump works more like a compressor. Water enters through the low pressure inlet and is compressed by the impeller as it moves past the stator and into the nozzle. The low pressure inlet is always larger in square inches of space than the outlet nozzle.

The stator has a main purpose that most may overlook; probably wondering what it’s doing in there. But the boat can’t run without it. If it were not there, the torque from the impeller (compressor) would cause the boat to twist to the right from centrifugal force. The stator has a very aggressive blade design that almost completely reverses the direction of water flow. By changing the direction of the flow, it stabilizes the water for even output, removing the centrifugal force of the impeller. But more importantly, by changing that direction, it increases the energy of the fluid movement.

The last part of the pump is the diffuser. It’s that little vertical blade you see when looking into the pump from the back. It’s designed to remove any rotational force left over from passing through the stator.

At idle, the impeller (compressor) is completely submerged and while maneuvering in no wake zones…water is sucked into the impeller through the intake grate and gently compressed for a small amount of thrust.
But as the engine is given more power and begins to move onto plane, the pump is literally lifted out of the water and the compressor is being supplied with high volume, low pressure water, which is then compressed at higher rates as it moves through the nozzle. Without the wearing rings close tolerance, the water would lap over the outer edges of the blade and back into the low pressure side, where it would just do circles. The wearing ring, being within thousandths of an inch away, ensures that all the compressed water makes its way into the stator, to increase the thrust by changing its direction. The faster you go, the more pressure you put on the low pressure side of the impeller.

This principle of operation is very similar to the way a steam and gas turbine work. They have a several sets of rotating blades (impeller) and several sets of stator blades.

In comparing our pumps to a gas turbine on a jet airplane, if you were to change the size of the inlet to that compressor, only by inches, it wouldn’t get enough ram air to fly. The same is true with us. If you catch debris, like a ice bag left floating in the water, you’d lose all forward thrust. You’d find yourself under the boat/ski to pull that trash out of the suction grate.

So, in conclusion, when you are moving at WOT and do a sharp turn, the lose of speed equals a lose of pressure on the impellers inlet side, which in turn is going to reduce forward thrust till you straighten out and begin to regain speed.

If your sitting at a dead stop and slowly increase the throttle, then you will slowly gain speed, supposedly without cavitating. If you nail the throttle to 7k rpm, your engine is likely to hesitate while the compressor is given time to pump up the nozzle for thrust.

In my personal opinion, by a logical conclusion, I think the pure power generated by the 4-TEC’s and the 951’s would see less time in this loss of power just from the shear horsepower these engines create.

Any input or questions to this theory is welcomed. I hope I haven’t caused any/much confusion but after hearing all these questions about cavitating on start up or turning, I felt the need to go over the principle of operation and theory of how a jet pump works.

EvilVtec

01-19-09, 08:34 AM

Im a little new to the forum, so I have just been cruising around the sticky posts to see what everyone has to say. That is a great write up, and the theory of cavitation should be researched more in depth by everyone who is interested in jet propulsion since it makes up a great deal of the vessels performance. One thing you left out was the actual physics of cavitation, which might explain it a little more to everyone. The boiling point of water at normal atmosphere is 100c (212*F)degrees (atmosphere is 14.7 lbs/in2.) When you change this pressure (low pressure side of pump) the boiling point will decrease in linear proportion to the measure of pressure, meaning that the water will boil off inside of the pump. When this happens, the water instantly becomes vapor, and the propulsion system is ineffective in the water (the pump will spin freely in air vapor since there is no solid water to push). Since the water as a volume itself cannot be introduced to pressure changes, this "boiling" only happens when air bubbles are injected into the system. That is why the seals on the jet pump shaft are so important. If these seals are not air tight, it will draw air into the pump and cavitation will occur. I have always loved the concept of jet pumps and how they perform. IMO there is no better way to power a vessel. A lot of people who are into performance will you that the one of the most important things you can do to a ski is improve the intake grate, which helps stop cavitation and in turn, the boat will run faster and stronger.